From the original application: Long projection axons regenerate poorly through the damaged central nervous system. One key reason is the environment through which they must re-grow contains many biochemical signals that trigger signaling pathways which inhibit axon outgrowth. Some of these inhibitory cues are repellents that play an important role in guiding axons to their appropriate targets during normal embryonic development. Our previous work has shown that activation of a specific G-protein coupled signaling pathway, for example by the chemokine SDF1 activating its receptor CXCR4, makes axons less responsive to several different repellent guidance cues. We have partially characterized this anti-repellent signaling pathway with pharmacological reagents in tissue culture experiments and with anti-sense reagents in zebrafish embryogenesis. The aims of this project are to: characterize additional signaling steps in the pathway, identify dominant-negative proteins that block steps in the pathway both in culture and in the embryo, and examine how this anti-repellent signaling pathway affects axon guidance during normal development. As some of the signaling components in this pathway are also found in the cannabinoid and opioid pathways, our studies may identify developmental events that are influenced by prenatal exposure to drugs of abuse. These studies will further our understanding of how multiple guidance cues are integrated into single decisions by growing axons during normal development and will likely suggest novel approaches to promoting regeneration in the mature central nervous system. For the supplemental revision written in response to NOT-OD-09-058 "NIH Announces the Availability of Recovery Act Funds for Competitive Revision Application": Neurotransmitters have been hypothesized to provide axonal guidance information in the developing nervous system. We will test whether the metababotropic GABAB receptor, signaling through the anti-repellent pathway we have discovered and characterized, is required for normal axonal pathfinding in vivo. PUBLIC HEALTH RELEVANCE: Spinal cord injuries leave approximately 10,000 people partially or fully paralyzed in the United States each year. The prognosis for these kinds of injuries is extremely poor since re-growth of long axonal processes in the central nervous system is very poor. This project discovered and is now characterizing a signaling pathway that makes axons resistant to repellent guidance cues like those that prevent axonal regeneration. This pathway has some components in common with the cannabinoid and opioid signaling pathways and may also identify developmental events that are sensitive to prenatal exposure to these drugs of abuse.